1. Field of the Invention
The present invention relates to the fabrication of integrated circuit devices on semiconductor substrates and, more particularly relates to methods for fabricating monitor structures on the substrates.
2. Description of the Related Art
The manufacture of large scale integrated circuits in a mass production facility involves hundreds of discrete processing steps beginning with the introduction of blank semiconductor wafers at one end and recovering the completed chips at the other. The manufacturing process is usually viewed as consisting of the segment wherein the semiconductor devices are formed within the silicon surface (front-end-of-line) and the portion which includes the formation of the various layers of interconnection metallurgy above the silicon surface (back-end-of-line). Most of these processing steps involve depositing layers of material, patterning them by photolithographic techniques, and etching away the unwanted portions. The materials consist primarily of insulators and metal alloys. In some instances the patterned layers serve as temporary protective masks. In others they are the functional components of the integrated circuit chip.
While most development has been directed towards the manufacture of semiconductor based electronic circuits, there has recently been considerable interest in integrating electromechanical devices such as electric motors, springs cantilevered devices, and mechanical switches and oscillators within these electronic circuits. The repertoire of material along with a variety of available deposition and selective etching methods which have been developed in the integrated circuit industry along with a refined patterning technology have made possible the manufacture of tiny mechanical devices with movable elements. A movable element, for example the rotor of an electric motor, is patterned of material deposited onto a sacrificial layer, within a stator element. The sacrificial layer is then removed by selective isotropic etching which undercuts the rotor, freeing it from the substrate. Cantilevered devices such as mechanical switches, tuning forks or other oscillators, and leaf springs are similarly formed partially over a sacrificial layer, with an anchored portion connected to a subjacent structure.
In order to monitor the integrated circuit manufacturing process, test structures, representative of the circuit elements are typically incorporated in regions of the wafer outside the integrated circuit chips. Examples of these inline test devices include a dumb-bell structure testable with a four point probe to establish proper resistance of a deposited layer, or long serpentine metal lines which can be tested to establish the presence of particulate defects by testing for electrical opens and shorts. These devices are often designed much larger than their corresponding elements in the integrated circuit so they can be tested at various stages during processing.
Typically, these test devices are formed in the saw kerf which separates the circuit chips. In some instances, the test devices are formed in a designated chip site, referred to as a test site. However, this is usually avoided because it utilizes valuable product chip real estate.
In a product, which has micro electromechanical systems (MEMS), it is also desirable to have representative test structures to perform timely in-line testing of these devices as well. A problem with forming electromechanical test structures in the wafer kerf, or even in test sites, is that considerable particulate debris can be generated by the fracturing of free standing or lightly attached elements of these test structures. The expression “lightly attached” is used herein to indicate a structural element, for example, a long cantilever with a relatively small region of attachment to the substrate, thereby rendering it easily broken off. It is therefore desirable to have test structures for electromechanical devices which are designed to provide useful in-line testing but not having free standing or even lightly attached elements like their circuit counterparts.
FIG. 1 is a schematic cross sectional view showing a prior art MEMS device. The MEMS device comprises a central bearing 124a, a rotor 120b and a stator 120a in the semiconductor structure area 115 of the substrate 110. It also comprises a monitor pattern 124b in the semiconductor monitor structure area 118. The polysilicon plate 16 serves as an electric shield. Before forming the pattern in FIG. 1, a relieving process is used to remove sacrificial layers (not shown) in the semiconductor structure and the semiconductor monitor structure. The relieving process usually is a wet etch process and may lift off the semiconductor monitor structure 124b if the semiconductor monitor structure 124b is not well anchored to the substrate 110. The lift-off of the semiconductor monitor structure 124b results in a particle issue that reduces the yield of the MEMS devices. In order to resolve the particle issue, the semiconductor monitor structure 124b should be well anchored to the substrate 110 by increasing the contact area between the semiconductor monitor structure 124b and the substrate 110. But when the semiconductor monitor structure 124b becomes complicated, the design of maintaining the contact area between the monitor structure 124b and the substrate 110 also becomes complicated. Such a design has increased inconvenience for fabricating the MEMS devices.
Further, process monitors in the saw kerf of the wafer, with non-anchored or lightly attached mechanical elements, would release significant (or more than normal) debris during wafer dicing. U.S. Pat. No. 5,668,062 shows that when integrated circuit chips contain mechanical devices, in this instance mechanical mirrors, that the chips cannot be protectively coated during wafer dicing. Steps must be taken to eliminate metal fragments in the saw debris and which would otherwise lodge under the movable mirrors. The solution taught by the reference involves defining scribe line extensions of the array scribe lines to the edge of the wafer, whereby the scribe line extensions as well as the array scribe lines are free of the metal which is used to form the mirrors. While the procedure is very narrow in scope, the reference nevertheless shows that a specific type of debris (aluminum flakes during the dicing operation) compromises the proper function of MEMS devices.
U.S. Pat. No. 6,337,027 B1 teaches the formation of MEMS devices which are formed from in an epitaxial layer on a sacrificial silicon substrate. The devices, still on the substrate, are then bonded onto pedestals on a glass substrate. The sacrificial silicon substrate is then removed by spray etching, leaving the individual devices mounted on the supporting pedestals. The reference includes several methods of encapsulation of the complete MEMS devices before the substrate is diced using laser scribing.
U.S. Pat. No. 6,150,186 teaches the coating of a metal wire spring bonded to a silicon substrate to form a more resilient spring. The coating method improves the mechanical properties of the spring. The coating method may also be used to improve the resiliency of other spring devices such as a cantilevered spring.
U.S. Pat. No. 5,660,680 cites procedures for forming various useful micro structures such as tubes and beams as well as micro sensing and actuating devices by the use of patterned sacrificial molds in which the devices are formed and thereafter released by etching away the mold.